The Impact of Glyphosate on Soil Health

THIRD WORLD NETWORK BIOSAFETY INFORMATION SERVICE

Dear Friends and Colleagues

The Impact of Glyphosate on Soil Health

Soils are the foundation of our food security and yet a recent global scientific assessment found that 33% of land is degraded due to the erosion, salinization, compaction and acidification and chemical pollution of our soils. Over the last decade, about 6.1 billion kilograms of the herbicide glyphosate have been sprayed on the world’s farms, gardens and public spaces.

The Soil Association has reviewed the scientific evidence on the impact of glyphosate on soils and soil life. The key findings are as follows:

Leaching into water: Glyphosate’s ability to leach deeper into the soil or groundwater, rivers or lakes was low due to its ability to attach to soil particles. In particular types of soil or weather conditions, however, glyphosate can leach out and pose a potential pollution threat to water courses.

Impact on soil micro-organisms that are vital to soil health: Glyphosate has been shown to negatively affect the abundance of the culturable bacterial community, and the total bacterial composition. For example, a study found that the relative abundance of Acidobacteria decreased in response to glyphosate exposure. Decreases in the abundance of these bacteria over the long-term could impair the ability of soil to perform certain biogeochemical reactions performed by them.

Impact on fungi:Beneficial fungi that live near plant roots have been found to be harmed by glyphosate. For example, studies found 40% reduction of mycorrhization after the application of glyphosate in soils.

Severity and occurrence of crop diseases:Using glyphosate has increased the severity or the re-emergence of crop diseases, potentially by changing the balance between beneficial and harmful microbes in the soil. Increased frequency of soil-borne pathogens and reduced ability of crops to defend against them, are both reported to result from glyphosate use. In one study, the colonisation of roots by Fusarium fungi increased steadily as soybean growth progressed and as the rate of glyphosate increased.

Impact on earthworms:Glyphosate showed a negative impact on the reproduction, growth, movement or activity of different species of earthworms. A study also found that earthworms avoided soil treated with the herbicide.

The report recommends further research to examine the impact of glyphosate on other soil fauna and microflora. Research should also consider the differences between the impact of glyphosate on soils as an active ingredient only, and when it is combined with other ingredients in a range of commercial products.

Over the last decade, about 6.1 billion kilograms of the herbicide glyphosate have been applied worldwide. Glyphosate [N-(phosphonomethyl) glycine] is an active ingredient in a range of weed killer products, created for use in agriculture, horticulture and at amenity sites. Its use globally has risen almost 15-fold since 1996, when genetically engineered glyphosate-tolerant “Roundup Ready” crops were introduced. In Great Britain in 2014, 1.9 million kilograms of glyphosate were used on agricultural and horticultural crops, on 2.2 million hectares.

Despite being the most heavily applied herbicide in the world, in 2015 glyphosate was classified as ‘probably carcinogenic to humans’ by the International Agency for Research on Cancer, the specialized cancer agency of the World Health Organization, following a review of evidence from human exposure studies and in research on laboratory animals.

Furthermore, in 2016, scientists raised their concerns about the safety of glyphosate-based herbicides and call for new investments in epidemiological studies, biomonitoring, and toxicology studies.

Whilst the effects of glyphosate on human health are coming under scrutiny, scientists are now concerned about our insufficient knowledge of the ecological safety of glyphosate, the way it behaves in the natural environment, how it interacts with living organisms, and the pathways through which it is degraded.

Glyphosate has been considered an environmentally safe herbicide because it is assumed to be inactivated quickly after spraying due to rapid sorption onto particles in the soil, and its fast degradation by microbes.

In addition, the mechanism by which it kills plants (inhibiting the shikimic acid metabolic pathway) is thought to be unique to plants and some micro-organisms, including bacteria, algae and fungi, and thus theoretically not a threat to mammals. However, evidence from several studies now shows that glyphosate-based herbicides, via multiple mechanisms, can adversely affect the biology of mammals. Furthermore, the half-life of glyphosate, which gives an indication of its persistence in the soil and water, is believed to be longer than previously thought. Recent research suggests that the herbicide persists longer with the return of crop residues containing glyphosate to the soil. There is evidence to suggest that glyphosate-based herbicides can adversely affect aquatic invertebrate ecology and research has also shown a negative impact on amphibian larvae (tadpoles) and earthworms.

As well as the active ingredient of glyphosate, there is also concern about impacts of the adjuvants (other chemical substances that are added such as solvents and surfactants) in commercial glyphosate products, where different formulations have been found to have different levels of toxicity compared to pure glyphosate.

The Soil Association has reviewed the science on the impact of glyphosate on soils and soil life. For the world’s most widely sold weed-killer, we found surprisingly little research has been done. What research there is shows contrasting results, significant uncertainty and some evidence that glyphosate causes harm. More research is urgently needed.

Recommendations for future research

• Research should examine the impact of glyphosate on other soil fauna in addition to earthworms; nematodes, ants, beetles, termites, spiders, anthropods, molluscs and protozoa.

• Research should look at both the impact on specific species of soil fauna and microflora but also the ‘knock-on’ succession effects of changes in the soil ecosystem.

• Research should consider the differences between the impact of glyphosate on soils as an active ingredient only, and when it is combined with other ingredients in a range of commercial products.

• Many of the studies described in this report have looked at glyphosate in the context of its use in relation to genetically-modified glyphosate resistant crops. In the UK context, where GM crops are not commercially grown, it is just as important to consider the use of glyphosate in relation to non-GM crops, and in amenity situations.

• Research should consider whether there is a significant build-up of AMPA in soils, which is produced when glyphosate is broken down, and whether this is problematic or not, as it is considered mildly toxic to plants.